My son has EOSINOPHILIC GASTROENTERITIS, what should we do next?

DR,MY SON, MALE, 11 YRS OLD. ONE YEAR AGO, HE HAVING STOMACH ACHE. WE HAVE VISITED MANY CLINICS AND SPECIALIST. ALL THE TIME DIAGNOSE FOR NORMAL GAS PROBLEM. PAIN WILL GO AND COME BACK AFTER SOME TIME. ABOUT 3 MONTH AGO THE PAIN GETTING WORST. WHEN HE TESTED FOR BLOOD, THEY FOUNT HIS RED BLOOD CELL COUNT IS LOW. DR DID SCOPE AND TOLD US HE HAVING EOSINOPHILIC GASTROENTERITIS. HIS BLOOD TEST DOES NOT SHOW ANY ABNORMALITY AT WHITE CELL COUNT. THE SCOPING SHOW SMALL WOUND AT SMALL INTENSIL AND REDDISHNESS AT STOMACH. STOMACH PAIN IS THE ONLY SYSPMTOM HE HAVE, NO OTHER SIGN. HE DID NOT PUT WEIGHT PASS ONE YEAR AT 28KG. THE DR PRESCRIBE PREVISOLONE 40 MG FOR NEXT TWO WEEKS. PLEASE ADVICE WHAT WE SHOULD DO NEXT TO CONFIRM THIS. THANKS